The long-term goal of the collaborative research between the PI and the foreign collaborator is to determine the role of gonadal steroids on the sexual differentiation of brain and body during development and in adulthood. Sex differences in brain structure and function have been characterized in a number of places in several vertebrate species. However, it has been difficult to determine how genetic and epigenetic factors interact to result in sexual differentiation of the brain. Recent studies have suggested the potential importance of non-hormonal genetic mechanisms. The current aims extend the specific aims of the parent grant to determine the interaction of hormonal and genetic mechanisms for the generation of sex differences in adults. In SF-1 knockout mice the gonads regress early in embryonic development and so these mice are never exposed to endogenous gonadal steroids. Sex differences in their brains should arise only as a consequence of the different genetic background between males and females. We will use genetically disrupted mice that have been rescued from adrenal insufficiency to determine gonadal steroid dependent sexual dimorphism in the forebrain and establish the cause of sex differences in obesity in adult SF-1 knockout mice. Specifically we will examine brains from SF-1 knockout male and female mice and their littermate controls to determine what sex differences remain to account for the sex differences in activity and body weight. Brains from adult animals will be examined by histological, immunocytochemical and in situ hybridization procedures to characterize sex differences in brain structure and gene expression. Animals will be treated with either estrogen or androgen to assess the steroid responsiveness of brain circuits developed in the absence of gonadal steroids. These results will be critical from the perspective that the most widely held belief is that gonadal steroids are mostly, if not solely, responsible for sexual differentiation of the brain. The unique SF-1 knockout model system that the proposed experiments extend into adulthood will allow us to critically test the importance of gonadal steroids for brain sexual differentiation.